This invention relates in general to mounting heating elements on insulating materials and, more specifically, to mounting iron-chromium-aluminum alloy heater wire coils on ceramic fiber insulation walls.
Small furnaces, kilns, etc., have long been heated by electrical heating coils mounted on the interior of firebrick housings. While these kilns are initially inexpensive, the relatively inefficient insulation characteristics of firebrick have resulted in high energy costs, long cycling times and large, heavy kilns.
Some years ago, ceramic fiber insulation materials were developed. These ceramic fiber materials have one-fourth the weight and twice the insulating capability of firebrick. Initially expensive, the cost of ceramic fiber insulation has dropped due to improved manufacturing techniques, economics of higher volume production, etc.
As the cost of energy has increased, the ceramic fiber insulation materials have come into increasing use in small kilns, furnaces and the like. Problems have been encountered in mounting heating elements on the interior walls of such kilns. The method generally used is to form the fiber insulation entirely around and over the wire helix or coil used as the heating element. This total embedding is ordinarily accomplished during the vacuum forming of the ceramic fiber wall itself. To embed the heating coil less than 100% was found to produce a weak bond with the heater wires tending to work loose and crack the insulation.
While this full embediment method effectively held the heater wires in place, a number of problems remained. Because of the temperature gradient across the insulating material between the coil and the inner surface of the kiln, necessary to provide the desired interior temperature in the kiln, the heating elements often need to be as much as 400.degree. F. above the temperature of coils in open construction. These high temperatures greatly shorten heating element life. Of course, the inner insulation wall surrounding the coil is also overheated by a similar amount, which causes devitrification and shrinkage of the fibers with resultant severe warping and cracking of the insulation, thus reducing wall insulating properties. Also, since the volume of insulating material surrounding the heating coil must act as part of the heater, the overall thickness of insulating material must be increased to prevent the kiln exterior surfaces from reaching dangerously high temperatures.
Attempts have been made to anchor the bottom of each loop of the wire helix in a dense alumina bar, then embed the bar in a slot in the fiber wall. While this allows the exposed portion of the coil to radiate freely, this system is complex and costly to manufacture. Problems remain in obtaining secure bonding between the alumina bar and ceramic fiber wall and in retaining good bonding between heater coil and alumina during prolonged thermal cycling.
Thus, there is a continuing need for improved methods of mounting heater wire coils on ceramic fiber insulation walls.